feat: add ability to force 32bit epoch time

[milkpirate]

Summary

My ESP core seems to fill only the lower 32 bits of it 64bit time_t with the correct epoch value. But does not clear the upper bits leave garbage, which causes issues when the value is cast back to int64_t.

NOTE: Enableing ESPDATE_FORCE_32BIT_EPOCH will reintroduces the year 2038 problem.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (code change that neither fixes a bug nor adds a feature)
• Documentation update
• Build/CI

Details

• What does this PR change? Why? See description.
• Any context for reviewers to understand decisions and tradeoffs. No.

Testing

• Builds locally
• Unit tests added/updated
• Tested on target hardware/device (if applicable)

Describe how you tested the changes. Include commands, logs, or screenshots if helpful.

[vars]
serial_speed = 115200

[platformio]
core_dir = .platformio_core/
default_envs = pico32

[env:pico32]
platform = espressif32 @ ^6.13.0
board = pico32
framework = arduino
upload_speed = 921600
monitor_speed = ${vars.serial_speed}
platform_packages =
	espressif/toolchain-xtensa-esp32 @ ~12
build_flags =
	-std=gnu2x
	-std=gnu++23
	-DSERIAL_BAUD_RATE=${vars.serial_speed}
	-DESPDATE_FORCE_32BIT_EPOCH
build_unflags =
	-std=gnu11
	-std=gnu++11
lib_deps =
	https://github.com/milkpirate/esp-date.git#feat/add-ESPDATE_FORCE_32BIT_EPOCH-option
debug_tool = esp-prog

void loop() {
    auto now = date.now();
    Serial.println(now.epochSeconds);
    Serial.println(now.localString().c_str());
    delay(1000);
}

Without -DESPDATE_FORCE_32BIT_EPOCH:

1862973903003830
241157-08-02 11:23:50
1862982492938423
241429-10-16 00:20:23
1862973903003832
241157-08-02 11:23:52
1862969608036537
241021-06-26 04:55:37
1862969608036538
241021-06-26 04:55:38
1862973903003835
241157-08-02 11:23:55
1862973903003836
241157-08-02 11:23:56
1862973903003837
241157-08-02 11:23:57
1862973903003838
241157-08-02 11:23:58
1862973903003839
241157-08-02 11:23:59
1862973903003840
241157-08-02 11:24:00
1862969608036545
241021-06-26 04:55:45

with -DESPDATE_FORCE_32BIT_EPOCH:

1773592930
2026-03-15 17:42:10
1773592931
2026-03-15 17:42:11
1773592932
2026-03-15 17:42:12
1773592933
2026-03-15 17:42:13
1773592934
2026-03-15 17:42:14
1773592935
2026-03-15 17:42:15

Compatibility / Breaking Changes

• Describe any API, ABI, or behavior changes. None.
• Migration steps for users (if any). None

Risks and Mitigations

• Potential risks introduced by this change and how they’re mitigated.
  Warning was given via preprocessor if enabled.

Checklist

• [?] Code follows project style and conventions
• All tests pass locally
• Documentation updated (README, examples, comments)
• No secrets, credentials, or sensitive data committed
• Linked issue(s) and relevant context provided

Environment

• Toolchain/SDK versions: (e.g., ESP-IDF/PlatformIO/GCC)
• OS and version: see above
• Other relevant environment details: none